On all common distros (Debian, openSUSE, Ubuntu, Fedora, Mint...), at installation time you can only choose encryption if you choose to have a LVM setup. But why? Why can't you set a LUKS just on the partition you want without using a LVM setup?

You can, the problem is now detecting what is a crypt volume and what is not while booting the system, LVM works with LUKS so well because LVM contains everything in a primary volume which then can be encrypted to make everything under it (all partitions encrypted). if you used LUKS on a raw disk how would the boot process unlock the drive? INIT or SYSTEMD have no clue what devices you have, they look at the fstab to mount... problem is adding luks to find and open every volume would be another step... if you really wanted to just rewrite the startup script so it does do it... not really a big deal. Its just not prepackaged, like a lot of things.

I just tried the Fedora 20 installer, and it let me create ordinary partitions with encryption, the only restriction being that the /boot partition cannot be encrypted.

That is typically what needs to be done. The /boot partition needs to be the source of truth for how to start the system (including decryption) on a normal partition. The same applies when you're booting your system from software RAID (mdadm). The boot partition has to be on regular non-RAID disk so that it initializes the RAID and boots.

Throwing LVM in the mix does not change that requirement. I believe it is simply a design choice of the maintainers.

Actually, GRUB 2 can handle an encrypted /boot, but I don't know of any installers that set up the GRUB configuration with "GRUB_ENABLE_CRYPTODISK=y". Here is a link to one person's success story.

This is one of the reasons we use an initrd. For something like cryptsetup, initrd.img will be updated with the UUID of the encrypted volume. No need to check every volume.

Doesn't Ubuntu installer suggest the ecryptfs encryption, only on the /home mounted fs? ecryptfs doesn't need LVM because it's a filesystem encryption.
LUKS (that is device level encryption) neither needs strictly LVM as far as I know. I've an encrypted Slack installed on common partitions but the root partition needs to be unencrypted (maybe is the boot partition that strictly needs to be in clear?).
My schema is
/ clear
/tmp light encryption (password1)
/home more strong encryption. (password2)

I think that for my security needs (protecting data from hypothetically stolen notebook) having the root in clear does not imply a big risk. So I've not structured my disk with LVM.

have a nice day
Gio

only boot partition need to be unencrypted. Of course you can use LUKS on any filesystem, as it encrypts at block level. In fact, I use LUKS to encrypt stand-alone ext4 and NTFS partitions, and also a LVM.

But my question was why most distros only offered encryption if one decided to go for a LVM.However, with the latest versions this seems to be changing, as rknichols said.

Just a question - if the boot partition gets corrupted is there any way to decrypt the data?

If you use a passphrase to unlock the data, then any rescue or installation CD/DVD that supports LUKS (and LVM, if you used it) can be used to unlock and access the partition. If a key file is required, you would of course have to have a copy of that key file available. I've used SystemRescueCD to access LUKS encrypted LVM volumes. In fact, I verified that I could do that before setting up a system that way.